The present invention relates to a single resist layer lift-off process for forming patterned layers on a substrate. The process provides a patterned resist layer having improved resist profile overhang and negative slope in the sidewalls of the resist profile, which collectively increase production yield and reduce production time and expense.
Lift-off processing is often referred to as an "additive" process in contrast to the more typical use of a photoresist mask where the areas of desired metallization are protected and the undesired areas are left exposed and etched away using appropriate chemicals. In a lift-off process, the substrate is covered by photoresist everywhere except in areas where the metallization is desired. The metal is then added, covering the entire substrate with the metal sitting on top of the photoresist and in contact with the substrate in the open areas. The photoresist is subsequently removed lifting the unwanted metal away from the substrate, leaving behind the desired metal pattern.
The impetus for the lift-off process was the need to pattern metal lines on substrates where the use of chemical or plasma etching is either undesirable or incompatible with the process or materials involved. An example of this is in the processing of GaAs substrates. Typical metallization schemes require the use of a metal composites to form contacts and transmission lines. Typical metals used are aluminum, gold, nickel, chromium, platinum, tantalum, titanium and others, where the required contacts may use two or three layers of these metals in some combination. Etching these metals would require very harsh chemicals that would severely attack the GaAs substrate and degrade the performance of the device.
The other primary need for lift-off processing is when tight line width control is required. Typically, a wet chemical etch is isotropic in nature. Due to processing related deviations, metal films typically have thickness variations across the wafer. Film thickness variation requires the wafer be "over etched" in order to assure that complete etching has occurred. This translates into line widths being reduced when the isotropic etch works under the resist mask. The most severe case occurs where the film thickness is at a minimum. Because the lift-off process depends only on the control of the photoresist, line width is maintained independent of metal thickness or variations in the etch process.
The lift-off process was first introduced as a "brute force" processing technique. The idea was to deposit a thin metal coating 3 (.about.0.2 .mu.m) over a thick (.about.2 .mu.m) photoresist pattern 2, and then force the metal to make a clean break as shown in FIG. 1. Unfortunately this idealized process is not practical. First, the process requires the metal to be delivered normal to the surface of the substrate 1. The best approximation to this situation would occur in an evaporator. The metal "melt" is kept in a water cooled crucible inside an evacuated chamber (typically in the 10.sup.-6 to 10.sup.-8 Torr range) where an electron beam is directed via a magnetic field to impinge on the metal surface, heating the metal in a controlled manner. Due to the elevated temperature of the metal melt, a vapor pressure is created allowing the metal atoms to diffuse throughout the chamber, as if from a point source.
If the substrates are suspended at a sufficient distance, and aligned tangent to the parabolic curve of a dome, the metal atoms tend to arrive normal to the surface. In reality, certain constraints tend to limit the realization of this phenomena. Wafer surfaces tend to be quite large, ranging from 3 to 8 inches in diameter, so only a small portion can be tangent to the required curve. Practical considerations also tend to limit the source to wafer distance to under a meter with machining tolerances and quality control in the assembly of commercially available equipment, all making an ideal evaporation impossible to achieve on a production scale.
In reality what happens is the metal is delivered to the substrate at an angle (FIG. 3), and thus the metal will build up on one of the walls of the photoresist as shown in the SEM photographs of FIG. 2a and 2b (FIG. 2b being magnified photographs of each end of FIG. 2a). When the photoresist is subsequently removed, this buildup 3a can remain, a condition known as "winging" or "wing tip" (FIG. 3). Winging metal tends to curl, but remain connected to the deposited metal layer, to short adjacent lines of the metallized pattern, reducing yield (FIG. 4). Still other metallization schemes tend to be worse. Sputtering, which is more conformed than evaporation and chemical vapor deposition of metallic films, will completely cover the photoresist and impede its removal. If the photoresist overhang profile is poor the deposited metal will adhere together with the photoresist sidewall making it difficult to perform lift-off and resulting in rough metal line quality.
The second major obstacle to the brute force approach is the resist profile itself. Because photolithography is an optical process, it is subject to optical constraints. The light (board band or monochromatic) will be absorbed as it passes through the resist layer causing the top of the film to receive a higher dose of energy than the bottom, thus making the top layer of the resist more soluble in the developer. This produces resist profiles that instead of being at right angles to the substrate, form more rounded profiles, larger at the bottom and smaller at the top, defined as a positive slope as shown in FIG. 5a.
In addition to absorption, diffraction also plays a significant role in creating positive slope in the sidewalls of the resist profile allowing the light to spread out and expose a larger area at the surface of the resist. Diffraction effects become greatest when the mask is not in physical contact with the resist, a typical requirement in order to avoid damaging the mask and the wafer. As the gap between the mask and resist increases, the profile becomes quite rounded (FIG. 5b) and resolution is reduced.
In order to bypass these limitations and develop a controllable, repeatable lift-off process, a variety of processes have been developed in order to modify the positive resist profile or to develop complex double- or triple-layered structures whose primary goal is to create an overhang or undercut profile. IBM, in the summer of 1980, introduced the first single resist layer lift-off process that employed resist profile modification. See M. Hatzakis et al., "Single-Step Optical Lift-Off Process", IBM J. Res. Develop., Vol. 24, No. 4, July 1980; R. Halverson et al., "The Mechanism of Single-Step Lift-off with Chlorobenzene in a Diazo-Type Resist," IBM J. Res. Develop., Vol. 26, No. 5, September 1982; and G. Collins et al., "Process Control of the Chlorobenzene Single-Step Liftoff Process with a Diazo-Type Resist", IBM J. Res. Develop., Vol. 26, No. 5, September 1982.
In the IBM process, the positive photoresist is exposed in the usual manner and then soaked in an aromatic solvent, typically chlorobenzene. It is known that the penetration of the solvent into the resist, which defines the depth of the overhang profile, is controlled by the soaking time, solvent content after soft bake, developer concentrations, temperature and impurities in the chlorobenzene. See JP 60-32047 and Chem. Abst. 103-62591. During development of the resist, the previously exposed areas of the resist, where penetrated by chlorobenzene, tend to dissolve slower than the unpenetrated areas. Thus the unpenetrated resist is over-developed, resulting in an undercut resist profile, shown theoretically in FIG. 5c.
The IBM process is shown in flow diagram form in FIG. 6. Although this process has been widely adopted in industry, in production it is very difficult to control. Tight process controls must be observed during soft bake, soak, development and exposure. A small variation in any one of the many variables present in each of these steps will result in reduced production yield, increased production time and increased production expense. Additionally, when the resist is exposed before the chlorobenzene soak, different regions of exposed and unexposed resist have different solubilities to the chlorobenzene. Consequently, it is difficult to diffuse the chlorobenzene uniformly over the surface of the resist.
Other perhaps more significant problems with the IBM process is that the length of the resist profile overhang is insufficient and the sidewalls of the resist profile have positive slope (i.e., they curve into the resist) as shown in FIG. 5c. These problems result in metallization of at least part of the sidewalls of the resist (FIGS. 2a-2b). Such sidewall metallization also leaves wings (3a, FIG. 3) of metal which short adjacent metal lines of the deposited pattern. Such sidewall metallization (FIGS. 2a-2b) also (i) inhibits contact between the acetone and the resist during lift-off and (ii) anchors the resist to the substrate. Accordingly, it can take up to 8 hours for lift-off to be completed.